The present invention relates to electronic circuits and, more particularly, to electronic circuits for sensing temperature.
In the electronics industry, there is a need for protecting circuits from conducting too much current. It is well understood that the power dissipated in a circuit is equal to current squared multiplied by resistance. Power dissipated in the circuit is directly proportional to heat generated. Therefore, one way of detecting the amount of current passing through a circuit component is by measuring the temperature of the component.
One solution for protecting circuits from conducting too much current is to place a temperature sensitive current limiter in series with the circuit. One such current limiter commonly used is built on a discrete component and employs a polymer sandwiched between two copper plates. Current passing through the limiter heats the polymer and causes it to expand. This expansion leads to a greater resistance which leads to even more heat being generated. In a relatively small current swing, the resistance of the polymer goes from a low value (approximately 50 mxcexa9) to a large value (much greater than 1 kxcexa9). This effectively creates an open in the discrete component and limits power to the circuit to be protected. After the temperature in the current limiter has decreased, the polymer contracts and the resistance decreases to the mxcexa9 range again.
There are several disadvantages to this device for limiting current. First, the current limiter is built using discrete components and is not integratable onto an integrated circuit chip. Second, the size of the discrete component is about 10 times that of an integrated circuit package. Third, the initial resistance of this device increases with use.
In accordance with the present invention, there is provided an apparatus for generating a ramp signal between two temperature values. The apparatus can be constructed on an integrated circuit and does not have to be a discrete component. The ramp signal can then be used in many different configurations. In one aspect, the ramp signal is inputted into a power regulator that supplies power to a circuit that needs to be protected. As the signal decreases, the power regulator reduces the amount of power reaching the protected circuit. When the signal reaches its minimum value, power is limited to a selected amount to the protected circuit.
In another aspect, the ramp signal is inputted into logic which outputs a constant first signal until a first temperature is detected and then outputs a constant second signal until a second temperature lower than the first temperature is detected. When the second temperature is detected, the ramp signal causes the logic to output the constant first signal again until the first temperature is detected. Such a configuration could, for example, be used to control a climate control module.
In another aspect, the invention provides for the various parameters of the ramp signal to be varied. For example, the apparatus can be readily modified to cause the ramp signal to occur between different pairs of temperature values to suit a particular application. The slope of the ramp signal can be modified to accommodate applications which require a faster or slower signal transition. If, for example, a circuit was more sensitive to heat generated from excess current, the ramp signal could be modified to quickly move to limit power to the circuit. With the appropriate powering source, the minimum and maximum values between which the signal goes can also be varied to suit a particular application""s requirements.
Because the invention can reside on the same integrated circuit as the circuit it is protecting from excess power, the temperature controlled current sources feeding the apparatus can be tightly coupled to the temperature of sensitive components on the integrated circuit and easily connected to the apparatus. This reduces costs in building electronic devices while increasing performance in detecting temperature changes and eliminating excess current problems.